One of the less well-known transatlantic linguistic differences. In British English, a flan is an open-topped shortcrust pastry-based pie or tart with a sweet or savoury filling - the latter being not greatly different from a quiche in my book, especially if eggs are involved - served as an entrée or a dessert; the word comes from the Old French flaon and the Latin flado, meaning a flat cake. sneff informs me that this is also the normal meaning of the word in Australia (other than in Spanish restaurants) and draws my attention to the Ibizan Easter speciality flan de pascuas which manages to combine both paradigms.
The American usage is completely unfamilar to most Brits, but is noted by the Larousse Gastronomique as being often used in France and Spain for "an egg custard, often caramel flavoured, that is made in a mould and then turned out and served cold"; from the Mexican references in other writeups I presume that the word entered the American vocabulary from the Spanish. In francophone Belgium, the term is synonymous with an industrial crème caramel style dessert; cf the Dutch vla.